1. Field of the Invention
Embodiments of the invention relate to a semiconductor device such as a diode and insulated-gate bipolar transistor (IGBT) that has a field stop (FS) layer.
Among semiconductor devices are, for example, diodes and IGBTs that have breakdown voltage ratings of 400V, 600V, 1200V, 1700V, 3300V, or higher. These semiconductor devices are used in power converting equipment such as converters and inverters. Low loss, favorable electrical characteristics such as high efficiency and breakdown tolerance, and low cost are demanded of power semiconductor devices. In particular, better electrical characteristics and reduced unit chip cost can be obtained by a grinding technique of reducing the thickness of the semiconductor substrate to 200 μm or less.
FIG. 5 is a cross-sectional view of a structure of a conventional semiconductor device. FIG. 5 depicts a cross-sectional view of a typical trench IGBT. In FIG. 5, a p-type layer that forms a p-type base layer 54 is formed in a principal surface of an n-type semiconductor substrate that forms an n-type drift layer 51; and a p-type layer that forms a p-type collector layer 53 is formed in the opposite surface.
An n-type field stop layer 81 is formed in the n-type semiconductor substrate, at a position that is closer to the principal surface than the p-type collector layer 53 is. P-type guard ring layers 72 and field plates 73 forming a terminal region 71 for electric field relaxation are formed at a periphery of the p-type base layer 54. Further, gate electrodes 61a are aggregated and connected to a gate runner 65, and the gate runner 65 is connected to a non-depicted gate pad.
Among these layers, the n-type field stop layer 81 has to be of a depth and concentration necessary to prevent reach through of the depletion layer to the p-type collector layer 53. Reach through, for example, is a phenomenon where the depletion layer, which is the n-type drift layer 51 expanding, reaches a layer (p-type collector layer 53, etc.) adjacent to the n-type drift layer 51.
Conventionally, as a method of forming the n-type field stop layer 81, for example, an n-type impurity such as phosphorus or arsenic is irradiated from the ground back surface of the wafer and annealing is performed at a suitable temperature. However, the n-type field stop layer 81 is difficult to form at a deep position inside the substrate by this method.
N-type impurities such as selenium and sulfur have a diffusion coefficient that is higher than phosphorus and arsenic and therefore, diffuse about 30 μm at a temperature of 900 degrees C. Consequently, use of an n-type impurity such as selenium or sulfur in place of phosphorus or arsenic, enables the n-type field stop layer 81 to be formed deeper at a lower temperature.
A method of forming the n-type field stop layer 81 by hydrogen related donors is further known. A hydrogen related donor causes a vacancy-oxygen-hydrogen (VOH) defect to act as a donor. A VOH defect is formed by implanting hydrogen (proton, deuteron, triton, etc.) into a silicon substrate that includes oxygen and performing annealing at a temperature less than 500 degrees C., whereby vacancy (V), oxygen (O), hydrogen (H) are bonded, forming the VOH defect. A process that electrically activates implanted hydrogen to obtain hydrogen related donors can be realized by annealing at a relatively low temperature of 400 degrees C. Therefore, in the manufacturing of a thin diode or IGBT, the number of processes subsequent to the thinning of the wafer can be significantly reduced.
According to another method, proton implantation is performed multiple times to form multiple n-type field stop layers and these n-type field stop layers are caused to equivalently act as one broad n-type field stop layer.
Japanese Patent Application Laid-Open Publication No. 2013-138172 describes an IGBT in which an n-type field stop layer is formed by irradiating protons accelerated to a high energy of 4 MeV or greater.
FIG. 4 of U.S. Patent Application Publication No. 2008/0054369 describes an IGBT in which selenium is diffused as a first n-type field stop layer and inside the first n-type field stop layer, protons are irradiated to form a second n-type field stop layer.
FIG. 1 in International Publication No. 2012/157772 describes an IGBT in which selenium is diffused as a first n-type field stop layer, and phosphorus is implanted so as to form a second n-type field stop layer between the first n-type field stop layer and a p-type collector layer.
Nonetheless, selenium has a larger diffusion coefficient than phosphorus and arsenic to perform interstitial diffusion. Thus, compared to phosphorus and arsenic, adjustment of doping concentration distribution of the n-type layer is difficult even when the diffusion period and temperature are varied. For example, the diffusion depth of selenium cannot be adjusted very well to be less than or greater than 30 μm by varying the diffusion period and temperature. Therefore, with the conventional techniques above, a problem arises in that it is difficult to control of the n-type field stop layer to satisfy required device properties.
Further, with the conventional technique of performing proton implantation multiple times to form equivalently an broad n-type field stop layer, implantation has to be performed multiple times with an accelerated ion energy on the order of 1 to 10 MeV and thus, a problem arises in that cost increases consequent to the large scale of the accelerator and the necessary measures against radiation.
Further, as a result of earnest research by the inventor, the following problem was found to occur when high-energy accelerated protons are irradiated as described in Japanese Patent Application Laid-Open Publication No. 2013-138172. That is, since the silicon is significantly damaged (so-called disorder) by the high-energy accelerated proton, in addition to a reduced lifetime of the carriers in the irradiated portion, carrier mobility also decreases. Reduced lifetime or mobility leads to increased electrical loss.
As a countermeasure, when crystalline damage consequent to the irradiation of high-energy accelerated protons is to be reduced by increasing the temperature of annealing by electric furnace (hereinafter, furnace annealing), donors disappear above a predetermined temperature. Therefore, a problem arises in that it is difficult to generate hydrogen related donors at a high rate while achieving high mobility or long lifetime.